1. Field of the Invention
The present invention relates generally to a voice reproducing device, and more particularly, to a voice reproducing device used in an electronic apparatus for reproducing voice messages, voice guidance or the like.
2. Description of the Prior Art
When encoding a voice signal for storage in a semiconductor memory, if conventional pulse code modulation (PCM) is used, it is not possible to make a long recording of a voice message because of the limited capacity of the expensive semiconductor memory. Therefore, a method called Adaptive Differential PCM, or ADPCM, which provides enhanced data compression efficiency, is often employed in a voice reproducing device.
In the ADPCM method, data compression is achieved by encoding only the differential information derived from the correlation between voice signal elements. When the differential information is encoded, the quantization width is adaptively changed to increase responsiveness to the original waveform, thereby improving the signal-to-quantization-noise ratio (SNR).
A typical voice reproducing device utilizing the ADPCM method may be constructed, for example, as shown in FIG. 3, so that digital data read from a data ROM 21 is transferred to both a quantization width controller 22 and an inverse quantizer 23. Based on the supplied digital data, the quantization width controller 22 controls the quantization width by gradually expanding it when the maximum quantization value continues and gradually reducing it when the minimum quantization value continues. The inverse quantizer 23 converts the digital data to linear-quantized data in accordance with the quantization width set by the quantization width controller 22. Since the digital data output from the inverse quantizer 23 represents the differential information of the original voice signal, the digital data is integrated by a digital integrator and is then converted to an analog signal by a D/A converter 25 to reconstruct the original sound of the voice.
However, in the above constructed voice reproducing device, the quantization width controller 22 requires the use of a complicated multiplier and adder. Furthermore, a high-precision resistor and operational amplifier are needed for the D/A converter 25. Moreover, because of the employment of ADPCM, the maximum attainable data compression ratio is one bit per sampling period.
Thus, typical prior art voice reproducing devices have suffered from complicated circuit configurations and a limit to the data compression ratio. Further, the high cost and complexity of the required components has made reducing the overall size and cost of the devices difficult.
It would be desirable to provide a voice reproducing device capable of improving the data compression ratio while simplifying the hardware construction.